The present invention relates to improved oxidative dehydrogenation catalysts and the process of oxidative dehydrogenation employing the improved catalysts. More specifically, the invention relates to modified zinc ferrite catalysts which are effective in substantially reducing the by-product carbonyl compounds, e.g., aldehydes, produced during the oxidative dehydrogenation of hydrocarbons.
Zinc ferrite catalysts have been employed in oxidative dehydrogenation processes to convert saturated and/or unsaturated hydrocarbons to more highly unsaturated hydrocarbons through removal of hydrogen from such hydrocarbons as are known in the art, for example, as shown in U.S. Pat. Nos. 3, 303,235; 3,303,238; 3,607,966; and 3,856,880. These catalysts have proven adaptable to commercial use and appear to be among the best of the various disclosed catalysts for oxidative dehydrogenation. However, zinc ferrite catalysts produce carbonyl compounds as impurities.
Improved processes using zinc ferrite catalysts for the preparation of unsaturated hydrocarbons such as butene, butadiene-1,3, isoprene and styrene are processes, whereby hydrocarbons such as butane, butene, isopentene or ethyl benzene are dehydrogenated at elevated temperatures in the presence of catalysts and oxygen. Superior results and yields of product are thereby obtained. However, the product streams contain not only the desired unsaturated hydrocarbon, but also various oxygenated compounds such as aldehydes and other carbonyl compounds. When air is used as the source of the oxygen, the effluent from the dehydrogenation reactor will contain large quantities of relatively noncondensable gases, such as nitrogen. The gaseous effluent may also contain varying amounts of steam. It is one of the principle objects of this invention to provide a process for the reduction of the carbonyl and other oxygenated compounds in the gaseous product stream containing hydrocarbons.
The oxygenated compounds are a serious contaminant in the unsaturated hydrocarbon product and must be essentially completely removed in order to have a product of suitable purity, e.g., a product having on the order of a few parts per million carbonyl compounds. The essentially complete removal of the oxygenated compound is quite difficult for several reasons. In the first place, the oxygenated compounds constitute only a very minor percentage of the gaseous stream to be purified. Normally, the carbonyl compounds will constitute less than 5 mol percent of the gaseous stream to be purified and more usually may constitute such as less than or up to 2.5 mol percent of the gaseous stream. Generally, the feed stream will contain at least about 10 ppm carbonyl compounds based on the other organic compounds, such as the hydrocarbons. The oxygenated compounds are therefore quite difficult to remove because of their low concentrations in the gaseous stream. In addition, the oxygenated compounds may be difficult to separate from compounds such as hydrocarbons, regardless of their relative concentration. Azeotropes may form between the oxygenated compounds and various hydrocarbons. For instance, an azeotrope is formed between acetaldehyde and butadiene-1,3. It is, therefore, an advantage of this invention to provide modified zinc ferrite catalysts and process having a reduction of carbonyl compounds produced in the oxidative dehydrogenation.
Prior processes have dealt with the separation of oxygenated compounds from oxidative dehydrogenation processes. In U.S. Pat. No. b 3,308,201 and U.S. Pat. No. 3,336,414, oxygenated compounds are removed by scrubbing with an aqueous composition. These processes have the drawback that the carbonyl compounds are transferred to a scrubbing water and must still be disposed of such as by biodegradation. According to U.S. Pat. No. 3,557,238, the carbonyl compounds are condensed with the steam from the reactor effluent, revaporized and fed back to the dehydrogenation reactor. Therefore, one advantage of this invention is to provide an oxidative dehydrogenation process suppressing carbonyl compounds. It is a further advantage to reduce the carbonyl compounds produced without significantly losing reactor product or significantly isomerizing any of the products. A particular feature is to provide a process which is particularly effective in reducing formaldehyde because formaldehyde is difficult to economically separate from aqueous compositions.